The Burn Ultimatum: Why You Need to Monitor & Control Leaf Surface Temperature

One of the biggest problems cannabis cultivators deal with is trying to keep their crop from growing too close to lights — but not so far away that plants are light deprived.

Most ceilings average about eight feet tall. Some strains of full-size photoperiod cannabis can grow indoors up to 8 feet — even higher if you let it. When your crop grows into the lights, expect brittle blades and flimsy foliage.

There’s a fine line between plants that are too close to grow lights and plants that are too far from grow lights. The variables of this fine line include:

The type of grow lights you’re using.

The type, degree and effectiveness of climate control devices in your grow room.

Cannabis plants need radiation wavelengths that stimulate photosynthesis and formation of cannabinoids and terpenoids. What they don’t want is too much heat radiation (present as infrared wavelengths) delivered relentlessly to their leaves.

Some growers might liken this to the sunburn that humans experience, but that’s not a perfect example because plant leaves are thin, fragile structures that lack the defenses human skin can utilize to ward off excessive radiation.

Indeed, human skin generates melanin to protect itself from excess radiation, especially the radiation found in harmful ultraviolet wavelengths. Cannabis doesn’t have that ability, although some cannabis research scientists believe increased resin glad development is a marijuana plant’s protective strategy for shielding its leaves from excessive radiation and insects.

Cannabis also protects itself from excessive heat with evapotranspiration, the plant equivalent of sweating. This process increases nutrient water cycling and can lead to further problems, especially if the grower is using a maximum-strength feed program.

When overheated plants consume more than normal amounts of high-ppm (parts per million) water, they run the risk of getting burned by fertilizers or growing too tall.

The bottom line is, if your plants get too close to grow lights, their leaves and buds will be burned.

Grow Room Temperature Vs. Cannabis Leaf Surface Temperature

The tricky thing about grow-light heat management is that even if your ambient grow-room temperature at canopy level is in the target range (of 74–86°F, depending on vapor pressure deficit and whether you’re adding CO2 to the atmosphere), and if your cannabis canopy is too close to hot grow bulbs or LED fixtures, then your leaves will burn.

Overheated leaves result in a stressed crop. Stressed plants have growth problems, produce thin, airy buds, and may have other problems that further reduce yield and crop value. Burned buds are ruined, can’t be salvaged, and are a waste of crop inputs.

Overall grow-op temperature is always important, but the more crucial temperature to monitor and control is specifically leaf surface temperature.

I recommend investing in a specialized infrared thermometer designed to read leaf temperatures. Beware of el cheapo infrared versions or ones that aren’t intended for agricultural uses, because they read too wide a target area and are often inaccurate.

When I first used my infrared thermometer to measure leaf temperatures, I discovered that in grow phase and under single-ended metal halide grow bulbs, leaf surface temperature, or LST, was within 1–3 degrees of ambient grow room temperature measured at canopy level.

Measuring at canopy level is a challenge in itself. Your cannabis crop canopy changes in height as plants grow, but most growers have a stationary ambient temperature gauge or sensor. This stationary device might not be measuring the actual ambient temperature at the top of the canopy nearest the grow lights.

It isn’t just thermometer or temperature monitoring height placement that can influence grow-room ambient temperature readings. If the thermometer is directly in the path of grow light radiation, near an electric ballast, in the shade, or close to a fan or air conditioner, then the thermometer readings are unlikely to be accurate.

My solution is to attach my ambient grow op thermometers to light leashes, and continuously adjust height as plants grow taller so the thermometer is even with the top of the plant canopy. And I make sure not to place thermometers in the draft flow of a fan or air conditioning outflow, or where they will heat up by absorbing direct grow-light radiation.

Once getting the height placement perfect, what I discovered using my LST infrared thermometer is that certain types of LED grow lights and bloom-phase high intensity discharge grow lights generate massive doses of infrared radiation that increase leaf surface temperature.

This is most likely a problem during bloom phase when you’re using targeted grow lights, because infrared segments of the radiation spectrum are often accentuated in grow bulbs and LEDs designed for bloom phase, when plants use red and orange wavelengths as a means of boosting bud size.

The leaves of this cannabis plant have yellow patches, indicating they’ve been burned by the light.

How To Adjust Leaf Surface Temperature

With some types of LED grow lights and HID grow bulbs, and especially with double-ended HID grow lights, my LST was 5–10 degrees higher than ambient air temperature. I could see my cannabis leaves curling down, frying, with yellow and white flashing on the leaf tips. This alarmed me, so here’s what I did next:

Raised the grow lights.

Changed the type of grow lights I was using.

Utilized cooled or sealed grow lights so bulb heat is removed from the grow op before it enters the ambient airspace.

Moved the tallest plants out from under the most direct path of the grow-light footprint.

Added aeration fans to blow more cooled air through the crop canopy.

Bent, tied or otherwise manipulated my plants and/or their containers to lower their height, thus putting more distance between them and the infrared wavelength source.

Understanding Cannabis Leaf Surface Temperature

Let’s now explore the little-known nuances of monitoring and managing cannabis leaf surface temperature. Contrary to what many growers believe, merely increasing air conditioning capacity or lowering overall grow-room temperature doesn’t fix excessive LST, and those tactics may cause more problems than they solve, costing you more money, too.

There is no one-size-fits-all LST reading for every grow-room situation. If you’re adding CO2 in a sealed environment, you can run with higher ambient grow op temperatures, and this will influence LST target range.

Vapor pressure deficit is another crucial grow-room factor that, with the right climate, determines the correct LST for your grow op.

In general, I don’t like to see LST higher than 82°F or lower than 72°F. Trust your eyes. If your leaves look sunburned, if they’re curling away from the lights, if they’re distorted or turning brown, it could be that your leaf surface temperature is too high.

When you’re using your thermometer, take multiple readings from the topside of leaves, and on several leaves across the width and breadth of your plant canopy. You’ll often find significant differences in LST between leaves on plants located in different spatial proximities to your grow lights.

I recommend contacting your grow light and grow bulb manufacturers to ask about the infrared output of their products as it relates to leaf surface temperature and heat stress.

Finally, there is a low-tech, intuitive way to get an idea of your cannabis leaves’ surface temperature and infrared radiation. Hold the back of your hand under your grow lights at the same height as your crop’s leaves. If your hand feels too hot, like it would feel at noon on a summer day when you better slap on the 75 SPF lotion or you’ll be burnt to a crisp in minutes, then your plants’ leaves are suffering and you should act accordingly.

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